Indirect electrodeposition of a NiMo@Ni(OH)₂MoO_x composite catalyst for superior hydrogen production in acidic and alkaline electrolytes

Hydrogen production from renewable energy urgently needs application-oriented catalysts. In this work, a coupled procedure of indirect electrodeposition and in situ electrochemical activation is presented to synthesize a NiMo@Ni(OH)₂MoO_x composite coating on a graphite substrate. A mixed oxide coating of Ni(OH)₂ and MoO₃ is indirectly electrodeposited from a weak acidic citrate plating bath by means of local pH increase at the electrode interface caused by the hydrogen evolution reaction (HER). Without the need of special treatment, low valence of Ni⁰, Mo⁰ and MoO₂ are naturally formed and dispersed in the oxide matrix at the initial stage of the HER process in 1.0 M H₂SO₄ or 1.0 M KOH, leading to a significant activation of the catalyst coating. The final catalyst can achieve a HER activity comparable or even superior to Pt-based electrocatalysts in both acidic and alkaline solutions. Density functional theory calculations support a bimetallic catalytic mechanism involving NiMo active centers, which need a Gibbs free energy of atomic hydrogen absorption smaller than that on the Pt (111) surface. This work provides a facile and unique procedure for preparing highly active and industrially attractive electrocatalysts.